Zero crossing (or burst-firing) control is an approach for electrical control
circuits that starts operation with the
AC load voltage at close to 0 volts in the AC cycle.
This is in relation to solid state relays, such as
triac
A TRIAC (triode for alternating current; also bidirectional triode thyristor or bilateral triode thyristor) is a three terminal electronic component that conducts current in either direction when triggered. The term TRIAC is a genericised tradem ...
s and
silicon controlled rectifier
A silicon controlled rectifier or semiconductor controlled rectifier is a four-layer solid-state current-controlling device. The name "silicon controlled rectifier" is General Electric's trade name for a type of thyristor. The principle of four ...
s.
The purpose of the circuit is to start the triac conducting very near the time point when the load voltage is crossing zero volts (at the beginning or the middle of each AC cycle represented by a sine wave), so that the output voltage begins as a complete sine-wave half-cycle. In other words, if the controlling input signal is applied at any point during the AC output wave other than very close to the zero voltage point of that wave, the output of the switching device will "wait" to switch on until the output AC wave reaches its next zero point. This is useful when sudden turn-on in the middle of a sine-wave half cycle could cause undesirable effects like high frequency spikes for which the circuit or the environment is not expected to handle gracefully.
The point where the AC line voltage is 0 V is the z''ero cross point''. When a triac is connected in its simplest form, it can clip the beginning of the voltage curve, due to the minimum gate voltage of the triac. A
zero cross circuit
Zero crossing (or burst-firing) control is an approach for electrical control circuits that starts operation with the AC load voltage at close to 0 volts in the AC cycle. This is in relation to solid state relays, such as triacs and silicon contr ...
works to correct this problem, so that the triac functions as well as possible. This is typically done with
thyristor
A thyristor () is a solid-state semiconductor device with four layers of alternating P- and N-type materials used for high-power applications. It acts exclusively as a bistable switch (or a latch), conducting when the gate receives a current ...
s in two of the three phases.
Many opto-triacs come with zero cross circuits built in. They are often used to control larger, power triacs. In this setup triac turn-on delays will compound, so quick turn on times are important.
The corresponding
phase angle circuits are more sophisticated and more expensive than zero cross circuits.
References
External links
Simple Methods for Detecting Zero Crossing- IEEE
Isolated High Quality Mains Voltage Zero Crossing Detector
Electrical circuits
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